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Help people sending attachments larger than most email servers allow to send attachments via link and file sharing services.


Attachment Services

See the full list of other possible attachment services

For the Clippy prototype add-on we'll be initially developing against the following services, chosen for their fitting user experience and a useful API which we can test against.

Attachment Size Limits

via David Bienvenu

Each SMTP server may have its own limit - we get this information when we connect to the SMTP server so it is possible to persist this value the first time we connect to the SMTP server. But we also have to consider the receiving SMTP server; likely we'd probably set a hard max limit (e.g. 50MB).

The other issues are that for calculating the size, you need to account for that fact that we will base64 encode the attachment, which more or less doubles the size, and if the message is signed/encrypted, it might triple the size. We don't know exactly until we construct the message, but we can make an educated guess.


The initial development of this feature will be implemented as an add-on to Thunderbird. Eventually a feature like this on could make it into the core experience of Thunderbird.


Watch this page for an official add-on release.


If you're interested in helping develop the clippy project check out the clippy source code available through our Mercurial server.


See all the clippy mockups
Compose with Attachment help
Compose with an Attachment uploaded and one in progress
Revised Clippy layout


See all sketches at Thunderbird/Clippy/Sketches
Your Attachment is Too Large
Choose an Attachment Service
Now send your files quickly as links